New Rule: no-trailing-spaces

### Rule details

Disallow trailing whitespace at the end of lines in Markdown content, except where it is used intentionally to create a hard line break.

In CommonMark, a line ending preceded by 2 or more spaces produces a hard break (`
`). Anywhere else, trailing whitespace has no effect on rendering.

This rule allows exactly two trailing-space counts and flags everything else:
- 0 trailing spaces: allowed (no effect, nothing to flag).
- 1 trailing space: incorrect — has no effect.
- 2 trailing spaces: allowed (intentional hard break).
- 3 or more trailing spaces: incorrect — exceeds what's needed for a hard break.

### What type of rule is this?

Warns about a potential problem

### Example code

```markdown

This line has no trailing whitespace.[0 space]

This line ends with a hard break.[2 spaces]
Next line.

This line has one trailing space.[1 space]

This line has three trailing spaces.[more than 2 spaces]
Next line.
```

### Prior Art

markdownlint - [md009](https://github.com/DavidAnson/markdownlint/blob/main/doc/md009.md)

### Participation

- [x] I am willing to submit a pull request to implement this rule.

### AI acknowledgment

- [ ] I did not use AI to generate this issue report.
- [x] (If the above is not checked) I have reviewed the AI-generated content before submitting.

### Additional comments

Disclosure: I'm a participant of [open source contribution program OSSCA](https://github.com/eslint-ossca)